1. Field of the Invention
The present invention relates to a radio apparatus and a gain control method, and more particularly to a radio apparatus carrying out Automatic Gain Control (abbreviated as AGC hereinafter) for a signal from a mobile terminal unit in a mobile communication system, and a gain control method for adjusting a gain convergence rate of AGC in the radio apparatus.
2. Description of the Background Art
In a mobile communication system (for example, Personal Handyphone System, abbreviated as PHS hereinafter) which has been rapidly developed in recent years, an approach to extract a signal from a desired mobile terminal unit through adaptive array processing in a radio receiving system at a base station is proposed in communication between the base station and the mobile terminal unit.
The adaptive array processing is processing to remove an interference component and correctly extract a signal from a desired mobile terminal unit by calculating a weight vector consisting of a reception coefficient (weight) for each antenna at a base station based on a received signal from a mobile terminal unit, for adaptive control.
The adaptive array processing requires a plurality of spatially distributed antennas, that is, an array antenna. In an array antenna, for example, formed of two antennas, an array output signal Y(t) is expressed as follows:Y(t)=W1X1(t)+W2X2(t)                where X1(t), X2(t) each represents a received signal at each antenna, and W1, W2 each represents a weight at each antenna.        
The received signal at each antenna is expressed as follows:X1(t)=H11S1(t)+H12S2(t)+n1(t) X2(t)=H21S1(t)+H22S2(T)+n2(t)                where S1(t) represents a signal from a desired mobile terminal unit and S2(t) represents a signal from an interference-causing mobile terminal unit.        
Here, H11 represents a propagation path property from the desired mobile terminal unit to antenna 1, and H12 represents a propagation path property from the interference-causing mobile terminal unit to antenna 1. H21 represents a propagation path property from the desired mobile terminal unit to antenna 2, and H22 represents a propagation path property from the interference-causing mobile terminal unit to antenna 2. In addition, n1(t) represents noise at a receiving system of antenna 1, and n2(t) represents noise at the system of antenna 2. The array output in this case is expressed as follows:Y(t)=(W1H11+W2H21)S1(t)+(W1H21+W2H22)S2(t)+W1n1(t)+W2n2(t).
Here, it is assumed that the weight that satisfies the following equation can be calculated:(W1H11+W2H21)=1 (W1H21+W2H22)=0.
Accordingly, the array output signal can be expressed as follows:Y(t)=S1(t)+n(t)                where n(t)=W1n1(t)+W2n2(t).        
Thus, the interference component can be removed and the signal can be received from the desired mobile terminal unit by calculating the appropriate weight through the adaptive array processing.
FIG. 11 is a functional block diagram functionally illustrating a radio apparatus that is provided for each antenna at a conventional base station carrying out the adaptive array processing using such a plurality of antennas and carries out AGC for a signal from a mobile terminal unit. FIG. 12 is a flow chart illustrating a gain control method for adjusting a gain convergence rate of AGC in such a radio apparatus.
First referring to FIG. 11, a signal received from an antenna 1 is amplified by an AGC amplifier 2, converted into an IQ signal formed of an In-phase component (I component) and Quadrature component (Q component) by a quadrature detector 3, and thereafter stored into memory 4.
IQ signal once stored in memory 4 is provided to a demodulation circuit 5. Demodulation circuit 5 also receives IQ signal from another antenna (not shown), performs the adaptive array processing as described above and demodulation processing and extracts the signal from each mobile terminal unit.
A reception level detection unit 6 obtains the reception level of the signal from IQ signal stored in memory 4. An amplitude value of IQ signal is calculated, for example, for eight symbols from 60th symbol of the received signal for each frame. The maximum amplitude value for these eight symbols is considered as the reception level for that frame.
A feedback data calculator 7 calculates feedback data that decides an amplitude ratio of AGC amplifier 2 in the next frame by the reception level obtained by reception level detection unit 6 and a step constant stored in memory 9. Here, when the reception level obtained by reception level detection unit 6 is represented by P_max, a prescribed ideal value is represented by P_ideal, and the step constant is represented by Step, the amount of change ΔFB from feedback data at the time of receiving the previous frame to feedback data at the time of receiving the next frame can be calculated by the following equation:ΔFB=(P_max−P_ideal)/2Step.
When the value of the feedback data at the time of receiving the previous frame is represented by FB, feedback data FB′ at the time of receiving the next frame can be expressed as follows:FB′=FB−ΔFB.
The feedback data calculated by feedback data calculator 7 is once stored in memory 8. The stored feedback data is read in the next frame and provided to a gain control input of AGC amplifier 2 to be reflected in AGC at the time of receiving the next frame.
Referring to FIG. 12, the gain control method for adjusting the gain convergence rate of AGC in the radio apparatus shown in FIG. 11 will now be described. It is noted that the following process is implemented by a Digital Signal Processor (DSP) of the radio apparatus in a software manner.
At step S1, the signal from the mobile terminal unit is subjected to quadrature detection. Here, an RXIF signal that is an intermediate frequency signal received from the mobile terminal unit is converted to an RXIQ signal subjected to quadrature detection.
At step S2, such a symbol is set in that the reception level of the signal received from the mobile terminal unit starts to be detected. An example is herein shown where the reception level is detected from 60th symbol of the received signal.
At step S3, it is determined whether the present symbol is in a symbol period in which the reception level of the signal from the mobile terminal unit is detected. For example, when the reception level is detected in the 8-symbol period from the 60th symbol to the 67th symbol, if the symbol in which amplitude will be calculated from now on precedes the 68th symbol, a process of calculating the amplitude in that symbol will follow, and if not, a process of calculating the feedback data will follow.
At step S4, the amplitude in that symbol is calculated. The value of squared I component of IQ signal is added to the value of squared Q component of IQ signal. Here, in order to simplify the process, the square roots of the resulting sum is not obtained.
At step S5, it is determined whether or not amplitude A calculated at step S4 is greater than the maximum amplitude A_max stored until now.
At step S6, if it is determined that A is greater than A_max at step S5, A_max is replaced by A.
At step S7, the symbol in which the amplitude is calculated is shifted by one in ascending order.
At step S8, the amount of change of the feedback data is calculated from the maximum amplitude value of the amplitudes from the 60th symbol to 67th symbol and from a fixed step constant stored in the memory.
At step S9, the feedback data in the next frame is calculated and the calculated feedback data is reflected in AGC at the time of receiving the next frame.
In the adaptive array processing, weights are calculated such that the power of interference component is small when the signals received from a plurality of antennas are multiplied by respective weights to be synthesized. Specifically, the operation of multiplying the weight means that the amplitude and phase of the signal received from each antenna are adjusted appropriately. Therefore, if the amplitude and phase information of the received signal suffers an error due to waveform distortion and the like, the adaptive array processing cannot fully function.
On the other hand, as to the input/output characteristics of general amplifiers, output at a certain level causes saturation, thereby creating a non-linear region. Therefore, in a case where the feedback data is calculated based on the fixed step constant as described above, for example, if the reception level of the signal from the mobile terminal unit is largely fluctuated by the effects of a distance between the base station and the mobile terminal unit, any possible obstacle, fading, and the like, the amplifier (AGC amplifier 2) at the base station enters into this non-linear region, leading to distortion of the received waveform, thereby resulting in that the adaptive array processing cannot fully function.